Frequently, narrow-band interference signals are suppressed in radio receivers with the aid of band-stop filters. Narrow-band band-stop filters are also called notch filters. In spread-spectrum transmission methods, in particular, notch filters can be used for suppressing narrow-band interference signals (e.g. in a direct sequence spread spectrum (DSSS) receiver). Due to the frequency spreading of the useful signal, a narrow-band notch in the (spread) useful signal does not have a significant effect on the useful signal after despreading.
The frequency of an interference signal is not known a priori. Since a wide-band filtering will render the useful signal unuseable, only narrow-band filtering can be considered. For this purpose, the frequency of the interference signal can be determined and the frequency response of the notch filter can be adapted correspondingly so that the (narrow-band) stop band of the notch filter hits the interference signal. Such radio receivers plus notch filters with adaptively adaptable stop band are known, e.g., from the American patent document U.S. Pat. No. 7,706,769. However, notch filters having a stop band adaptable (in accordance with an input) are expensive to implement.